The present invention relates to a novel top separator and a method for producing pulp, preferably sulphate cellulose, with the aid of continuous digester systems.
Environmental demands has forced our industry to develop improved cooking and bleaching methods. One recent breakthrough within the field of cooking is ITC(trademark), which was developed in 1992-1993. ITC(trademark) is described in WO-9411566, which shows that very good results concerning the pulp quality may be achieved. ITC(trademark) is mainly based on using almost the same temperature (relatively low compared to prior art) in all cooking zones in combination with moderate alkaline levels. The ITC(trademark)-concept does not merely relate to the equalization of temperatures between different cooking zones, but a considerable contribution of the ITC(trademark)-concept relates to enabling an equalized alkaline profile also in the lower part of the counter-current cooking zone.
Moreover, it is known that impregnation with the aid of black liquor can improve the strength properties of the fibers in the pulp produced. The aim of the impregnation is, in the first place, to thoroughly soak each chip so that it becomes susceptible, by penetration and diffusion, to the active cooking chemicals which, in the context of sulphate cellulose, principally consist of sodium hydroxide and sodium sulphide.
If, as is customary according to prior art, a large proportion of the white liquor is supplied in connection with the impregnation, there will exist no distinct border between impregnation and cooking. This leads to difficulties in optimizing the conditions in the transition zone between impregnation and cooking.
Now it has been found that surprisingly good results can be achieved when:
1. Keeping a low temperature but a high alkali content in the beginning of a concurrent cooking zone of the digester;
2. Withdrawing a substantial part of a highly alkaline spent liquor that has passed through at least the concurrent cooking zone; and
3. Supplying a substantial portion of the withdrawn spent liquor that has a relatively high amount of rest-alkali, to a point that is adjacent the beginning of an impregnation zone.
This leads to a reduced H-factor demand, reduced consumption of cooking chemicals and better heat-economy. Additionally, the novel method leads to production of pulp that has a high quality and a very good bleachability, which means that bleach chemicals and methods can be chosen with a wider variety than before for reaching desired quality targets (brightness, yield, tear-strength, viscosity, etc.) of the finally bleached pulp. This novel process is defined in more detail in our co-pending application PCT/SE97/00192.
The present invention relates to a preferred method and device for practising the above. In connection with the continuous cooking of cellulose containing fibre material, it comprises impregnation of the fibre material with an impregnation liquid in an impregnation vessel and cooking of the impregnated fiber material in a digester, the impregnation vessel and the digester being connected to each other by a transfer circulation, which, via a feed line, feeds the fibre material from an outlet end of the impregnation vessel to the top of the digester, which feed-line comprises a separator for separation of free liquid from the fibre material and, which via a return line, feeds separated liquid back to the outlet end of the impregnation vessel for use as transfer liquid for the impregnated fibre material. Cooking liquid, is added to the fibre material after separation of the free liquid, preferably in connection with the top of the digester, downstream said separation of liquid.
According to conventional technique for withdrawal of liquid from the cooking system, this is normally done directly from the withdrawal strainer of the digester itself. Alternatively in connection with a two vessel system the impregnation vessel, may be supplied with fresh cooking liquor and equipped with a screening device, from which some of the withdrawn liquid is transferred to a recovery plant, possibly after first having passed a flash cyclone. The use of such a screening device involves a considerable cost, due to a special construction of the impregnation vessel being necessary, assembly of conduits and installation of screens, blind plates, nozzles, a possible central line and different instruments in addition to labour for assembling, welding etc. In addition to this there are difficulties in optimizing the withdrawal at this point. Moreover the operating costs of such a screening device is not neglectable. Furthermore the addition of white liquor (fresh cooking liquid) within the impregnation vessel or in the transfer circulation line leads to difficulties in optimizing the process. Firstly when supplying to the impregnation vessel it can be difficult to achieve sufficient mixing of the added white liquor in the impregnation vessel, leading to varying levels of alkaline in different parts. Secondly different kind of wood chips may consume varying amounts of alkaline, making it more difficult to optimize the conditions in the impregnation vessel. It is even claimed that the above might have a bad influence on cost and the quality of pulp, since if a too high amount of alkaline exists in connection with the mechanical action of the outlet scraper might deteriorate fibre strength.
The object of the present invention is to improve and simplify the cooking department with respect to withdrawal and supply of liquid from the cooking system. This is achieved by the use of a new method in connection with a new separator, also leading to a simplified construction of the impregnation vessel with resulting savings in material and costs and to a better way of optimizing withdrawal and supply of liquid thereby also creating conditions for a better utilisation of the cooking liquid.
The device according to the invention is characterised by a separator for wood chips disposed in the feed line between an impregnation vessel and a digester comprising:
a screw feeder having an inlet end and an outlet end for feeding wood chips in an upward direction from the inlet end towards the outlet end of the screw feeder;
a rotatable shaft in operative engagement with the screw feeder;
a drive unit secured to the rotatable shaft for rotating the rotatable shaft;
a cylindrical screen basket enclosing the screw feeder; and
a liquid collecting space enclosing the cylindrical screen basket for separating a substantial portion of a free liquid, the liquid collecting space being in fluid communication with a return line connected to the outlet of the impregnation vessel; and
a distribution means for supplying a cooking liquid to the fiber material, said distribution means being positioned downstream of the collecting space in relation to the flow of the chips.
According to a further aspect of the invention, less than 100%, preferably less than 95% and more preferred less than 90% of the liquid which is separated from the fibre material in the transfer circulation is recirculated to be used as transfer liquid for the impregnated fibre material and/or to be re-used in connection with the impregnation vessel, either as a transfer liquid or as an impregnation liquid.
Further the apparatus according to the invention is characterised in that it comprises a connection, which stretches from the liquid chamber of the separator to a recovery plant, for withdrawal of a second part of the liquid which is separated by the separator from the cooking system.
According to a preferred embodiment of the invention, the mixture of fibre material and impregnation liquid is fed through the entire impregnation vessel, without liquid being withdrawn from the cooking via the impregnation vessel, besides which a second part of the liquid which is separated in the separator is transferred to recovery.
It is preferred that the second part of the withdrawn liquid is allowed to flash before the recovery.
The second part of the withdrawn liquid may suitably constitute at most 20 m3/ADMT of pulp and at least 0.5 m3/ADMT of pulp, preferably at least 2 m3/ADMT of pulp and more preferred at least 4 m3/ADMT of pulp. It is suitable that liquid is separated from the fibre material in a controlled amount, so that the fibre material contains at least 0.5 m3 free liquid/ADMT of pulp.
According to yet another preferred embodiment of the invention, cooking liquid is added to the separator after separation of liquid in order to be intimately mixed with the fibre material which is poor in liquid, by influence of an upwards feeding screw in the top separator.
The said second part of liquid is suitably withdrawn from said return line via a branch line, directly or indirectly, outside the fibre material to the recovery without any essential part thereof being recirculated to the digester.